1. Field of the Invention
The embodiments discussed herein relate to a semiconductor device and a method of manufacturing a semiconductor device.
2. Description of the Related Art
Insulated gate bipolar transistors (IGBTs) having a higher carrier concentration on a collector side of a drift layer than the drift layer and a field stop (FS) layer of the same conductivity type as the drift layer are conventionally known. The field stop layer has a function of suppressing the spread of the depletion layer from the emitter side to the collector side at turn-on and increasing the carriers remaining on the collector side. Further, disposal of the field stop layer enables control of the injection of carriers from the collector side to the drift layer and thereby, enables breakdown voltage to be maintained even when the thickness of the drift layer is reduced to lower the on-voltage.
As an IGBT having a field stop layer (hereinafter, FS-IGBT), a device including a field stop layer having a higher concentration peak than the carrier concentration of the drift layer and carrier concentration distribution in which the carrier concentration decreases from this concentration peak toward the collector side and the emitter side has been proposed (for example, refer to Japanese Laid-Open Patent Publication No. S64-082563 (lines 16 to 20 of lower right column of page 3, lines 5 to 11 of upper right column of page 4, and FIGS. 1, 2)). In Japanese Laid-Open Patent Publication No. S64-082563, the carrier concentration of the field stop layer is equal to the carrier concentration of the drift layer at the interface with the drift layer, and shows peaking from the drift layer interface toward the collector side and gradual decrease from the peak toward the collector layer.
As a method of forming a field stop layer having such carrier concentration distribution, a method of using the effect of proton donors to form a field stop layer is conventionally known. In this method, a layer of defects (vacancies (V)) is formed by proton irradiation, at a predetermined depth from a back surface of an n−-type semiconductor substrate forming an n−-type drift layer. Hydrogen (H) atoms irradiated on the defects and oxygen (O) atoms in the n−-type semiconductor substrate bond, producing complex defects (Vacancy-Oxide-Hydrogen (VOH) defects). The VOH defects act as donors (hereinafter, hydrogen donors) supplying electrons, whereby the layer of VOH defects functions as an n-type field stop layer.
In the semiconductor device above, by heat treatment after the proton irradiation, VOH defect density is increased, enabling the hydrogen donor concentration (VOH defect concentration) to be increased. An activation process for increasing the hydrogen donor concentration may be implemented by low temperature annealing of 400 degrees C. or less. Therefore, for example, in producing (manufacturing) a thin-type diode or thin-type IGBT for which the product thickness has been reduced by grinding, processes after the thickness of the semiconductor wafer has been reduced may be significantly curtailed. Further, a broad (wide carrier concentration profile in a depth direction) n-type field stop layer formed by n-type layers respectively having carrier concentration peaks at differing depths may be formed by performing proton irradiation multiple times with differing accelerating voltages (for example, refer to US Patent Application No. 2008/0001257).
Further, as a method of forming an n-type field stop layer by proton irradiation, a method of irradiating protons from a back surface of a semiconductor wafer, simultaneously irradiating two types of lasers of differing wavelengths, and forming an n-type field stop layer using the effect of proton donors; and thereafter, forming a p+-type collector layer in a region through which protons pass (hereinafter, proton transmission region) on a back surface side of the semiconductor wafer has been proposed (for example, refer to Japanese Laid-Open Patent Publication No. 2009-176892). In the technique described in Japanese Laid-Open Patent Publication No. 2009-176892, the carrier concentration of a portion (tail portion) of the n-type field stop layer near a pn junction with a p+-type collector layer is higher than the carrier concentration of the n−-type drift layer.